Abstract

Retinal light exposures can decrease melatonin production at night. Since nocturnal melatonin suppression by light has been implicated as an endocrine disruptor and linked to certain diseases, such as diabetes, obesity and cancer, it is important to be able to estimate a threshold light level needed to reliably suppress melatonin. The present study was designed to develop a working threshold for melatonin suppression from “white” light that might be experienced by people in their living environments. Twenty-eight subjects participated in two studies. In addition to dark, control nights, subjects were exposed to 8, 22, and 60 lux at the cornea (study 1, n=14) and to 60, 200, and 720 lux (study 2, n=14) of a “warm white” light source (correlated color temperature of 2670 K). These corneal illuminance levels translate to modeled melatonin suppression levels of 1%, 2%, 6%, 19%, and 42%. In both studies, participants came to the laboratory for four nights, separated by one week. One blood sample was drawn in dim light at midnight and another blood sample was drawn after 60-minute exposure to each light condition (including dark). Using an orthodox statistical criterion for a Type I error of α< 0.05, only the 200 lux and the 720 lux exposures produced significant suppression levels (19% and 37%, respectively). Based upon a criterion modeled suppression level of 5% for a “cool white” light (6500 K), a corneal light exposure of 30 lux for 30 minutes from “white” light sources used in most architectural lighting applications is proposed as a conservative hypothesized working threshold for nocturnal melatonin suppression.